Flue gas desulfurization removes sulfur dioxide from the flue gas emitted during some manufacturing processes, including for example, power generation using high sulfur coal as the energy source. One of the byproducts of flue gas desulfurization is calcium sulfite hemihydrate, which can be oxidized and crystallized to produce gypsum for use by wallboard and cement manufacturers. Ex situ oxidation and crystallization is one of several alternatives of gypsum production from flue gases that contain sulfur dioxide. Ex situ oxidizers convert calcium sulfite hemihydrate to gypsum (calcium sulfate dehydrate) via the following reaction:CaSO3·½H2O+½O2+3/2H2O=CaSO4·2H2O.
Oxidation of calcium sulfite to produce calcium sulfate occurs only in liquid phase, so the calcium sulfite solids and oxygen must be dissolved into a liquid or slurry (including water) for the oxidation process to occur. In a typical ex situ oxidation and crystallization system, calcium sulfite slurry is fed into the oxidizer, oxygen-rich air is sparged into the oxidizer, and the calcium sulfites react with dissolved oxygen to form calcium sulfates (i.e., gypsum crystals). The gypsum crystal solids are agitated to remain in the liquid long enough to grow to a sufficient size for commercial use. Once the gypsum crystals reach a desired minimum size, they precipitate out of the liquid and are collected.
Conventional ex situ oxidizers that produce gypsum from calcium sulfite hemihydrate typically pump air into the oxidizer tank via an elaborate grid sparger. The elaborate grid sparger performs two functions: (i) distributing air across the cross-sectional area of the oxidizer tank, and (ii) keeping suspended solids agitated and moving.
Such conventional grid sparger systems typically have some operational disadvantages. For example, conventional grid sparger systems may produce large air bubbles that are less efficient at mass transfer than small air bubbles. A sparger typically releases small bubbles into the bottom of the tank, and these small bubbles coalesce into large air bubbles by the time the air rises to the liquid surface. Oxidation of calcium sulfite hemihydrate to produce gypsum requires mass transfer of oxygen from the air bubbles to the calcium sulfite hemihydrates, so large air bubbles that have a relatively low surface area do not produce as high mass transfer or utilization of oxygen from the air as small air bubbles. Also, some conventional gypsum crystallizers rely on the sparged air to provide agitation of the liquid to keep the gypsum crystals suspended during crystal formation, so use of a conventional grid sparger to provide liquid agitation may require a larger volume of air than is required for oxidation of the calcium sulfite hemihydrate.
Also, such conventional grid sparger systems typically have some cost and maintenance-related disadvantages. For example, if an oxidizer produces relatively large air bubbles that have a low mass transfer rate, a larger, more powerful air compressor may be needed that costs more to purchase and requires more energy to operate than a smaller air compressor. When large air bubbles reach the liquid surface, they may erupt violently from the liquid, which may introduce excess vibration into the oxidizer, thereby potentially reducing the life of the oxidizer components. Conventional grid spargers also require an extensive support and anchoring system on the tank floor that restricts access to some portions of the oxidizer and is expensive to install. It would be desirable to have an improved apparatus and method for crystallization of gypsum from flue gas desulfurization.